The invention relates to an optical scanner, and more particularly, to an arrangement for compensating for a displacement of a scanning spot which results from an inclination from a reference plane of a multi-faceted rotating mirror used in light beam scanning apparatus.
An optical scanner generally used in facsimile systems, optical printers or the like comprises a light beam scanner which employs a multi-faceted rotating mirror. Such a scanner directs a collimated light beam from a light source to a facet of a polyhedron such as hexahedron or octahedron, which reflects and focuses the beam onto a scanning surface. Rotation of the mirror in one direction performs a scanning of the focussed light spot across the scanning surface.
FIG. 1 shows an example of the prior art optical scanner. Multi-faceted rotating mirror 2 is rotatably mounted on support shaft 1 for rotation in one direction. A collimated light beam from a laser (not shown) impinges on one facet thereof. After reflection by reflecting surface 2a of the mirror, the beam is passed through condenser lens 3, which comprises a spherical lens, and is focussed onto a scanning surface 4 disposed in the focal plane of the condenser lens 3 as a scanning light spot. As mirror 2 rotates, the spot travels across and thus scans the surface 4. For the convenience of subsequent description, the direction indicated by optical axis 5 will be referred to as X-axis, the direction along which the beam scan takes place as Y-axis, and the axis of shaft 1 as Z-axis.
In the optical scanner thus constructed, the rotation of mirror 2 at a high speed may cause an oscillation thereof, producing an inclination of the reflecting surface to cause the light spot to be offset from the normal scan direction. Where the scanning surface represents an original and is conveyed in the direction of the Z-axis, this produces an error in the line-to-line spacing which is usually referred to as a pitch error.
Considering this arrangement more closely with reference to FIG. 2, if the reflecting surface 2a has an angle of inclination .DELTA..theta. with respect to the Z-axis as a result of an error in the manufacturing process or an oscillation during the rotation of the mirror, the collimated beam from the light source will impinge on the reflecting surface 2a with an angle of incidence .DELTA..theta. and be reflected at an angle of reflection .DELTA..theta., so that the reflected beam will have an angle of 2.DELTA..theta. with respect to the X-axis when passing through condenser lens 3. Consequently, the spot will be displaced on the scanning surface 4 by a distance .DELTA.Z=2.DELTA..theta..multidot.f.sub.s displaced from the optical axis 5 where f.sub.s represents the distance between condenser lens 3 and surface 4 or the focal length at the lens. For a small angle .DELTA..theta., we have 2.DELTA..theta..apprxeq.tan 2.DELTA..theta., and hence EQU .DELTA.Z=f.sub.s tan 2.DELTA..theta.
Such disadvantage can be overcome by the use of a cylindrical lens which prevents a pitch error in the event the reflecting surface is inclined, as disclosed in U.S. Pat. application Ser. No. 190,024.
Such scanner is illustrated in FIG. 3. Referring to this Figure, a light beam from laser 16 is adapted to impinge on reflecting surface 12a of a multi-faceted rotating mirror 12. The scanner includes a first convex, cylindrical lens 17 disposed in the optical path extending between the laser 16 and the mirror 12 for focussing the impinging beam in the axial direction of shaft 11 which is associated with the mirror 12, or in the direction of Z-axis, and a second convex, cylindrical lens 18 disposed between the mirror 12 and a condenser lens 13, which is a spherical lens similar to lens 3 of FIGS. 1 and 2, for refracting the reflected beam only in the direction of the Z-axis or in a direction perpendicular to the scan direction. Mirror 12 is coaxially secured to the free end of shaft 11, which is rotatably supported by bearing 19. The opposite end of shaft 11 fixedly carries drive pulley 20, which is engaged by endless belt 23 which also extends around pulley 22 fixedly mounted on the output shaft 21a of drive motor 21, thus driving mirror 12 for rotation in one direction.
In the arrangement described, the beam impinging on reflecting surface 12a is focussed in the axial direction of shaft 11 or in the direction of the Z-axis by convex lens 17 which is disposed so that the reflecting surface 12a is located on the focal position thereof, whereby a light image on reflecting surface 12a will be a linear image which is perpendicular to the Z-axis. The reflected image passes through second lens 18, and is focussed by condenser lens 13 onto the surface 14 as a light spot in a manner similar to the prior arrangement as far as the scan direction or the direction of Y-axis is concerned. Like condenser lens 13, the second convex cylindrical lens 18 is disposed so that the reflecting surface 12a and the scanning surface 14 are located conjugate to each other as far as the Z-axis component of ray is concerned. In this manner, the image which is focussed onto the surface 14 represents a spot, the position of which in the direction of the Z-axis remains unchanged if surface 12a is inclined from Z-axis to result in an incidence of the linear image reflected thereby onto the second lens 18 at an angle to change the optical path along which it passes from the convex cylindrical lens 18 to the condenser lens 13. Thus, a pitch error which usually results from an inclination of the rotating mirror is substantially completely avoided.
However, in the optical scanner arrangement described above, it is necessary to focus a light image of a narrow line on the surface 12a. If the scanning surface 14 comprises a recording member of a low sensitivity which is scanned by a high output beam from argon laser having a power output exceeding 10 W, the incidence of the beam onto the reflecting surface 12a will cause a temperature rise at the spot where it impinges, disadvantageously causing a deformation or damage of the reflecting surface 12a or burning of foreign matters deposited thereon which result in a degraded reflectivity.